50 ILLINOIS BIOLOGICAL MONOGRAPHS [496 



The movement of the tail was not a lashing as in many forms, but a 

 vibration in which the middle region was the most active. The cercaria 

 kept catching hold with its acetabulum and extending its anterior end, 

 but was not able to take hold with the oral sucker, which even in the 

 older specimens was not fully developed. Sometimes when a cercaria 

 under observation was pressed lightly with a cover-glass it would catch 

 hold with the ventral sucker and the vibrations of the tail would cause 

 it to swing round and round on the sucker as pivot. 



Cercaria douthitti is a very small form, cylindrical in cross section, 

 slightly wider at the center and tapering from the acetabulum to the 

 posterior end to a width equal to about that of the tail. The body has 

 an average length in well extended mounted specimens of 0.19 mm. and 

 a width of 0.067 mm. The tail is bifid and even when contracted is 

 equal to about one and one-half the length of the body. The lobes form 

 less than one-third of its length and are definitely constricted from the 

 main portion, making the tail appear as if jointed (Fig. 55). The 

 main stem of the tail has an average length of 0.22 mm. and a width 

 of 0.025 mm., while the lobes have about half that width and an average 

 length of 0.089 mm. When the cercaria is alive the tail has a range 

 of variation from less than once to about twice the length of the body. 

 Underneath the thin cuticula and the muscle layers of the tail is a layer 

 of unicellular club-shaped glands ( ? ) , which lie close together and have 

 their ducts extending forward to open to the outside along the sides. 

 These glands were only seen in the living specimens. The central 

 core of the tail is composed of parenchymatous tissue, thru which flows 

 the caudal branch of the excretory system. The base of the tail fits into 

 a depression at the posterior end of the bodv, which is open ventrally 

 (Fig. 57). 



The oral sucker of Cercaria douthitti is proportionally large, meas- 

 uring on the average 0.057 mm. in length and 0.045 mm. in width. It 

 is a mass of embryonic cells which are separated from the surrounding, 

 tissue by a fibrous sheath, except that the ducts of the cephalic glands 

 pass thru to open at the anterior tip. There is no differentiation into 

 muscle fibers and no mouth or oral cavity is marked. Cercaria ocellata, 

 a European forked-tailed form which corresponds in structure very 

 closely to Cercaria douthitti is described as having a definite mouth 

 opening, by La Valette St. George (1855:22-23). He also gives measure- 

 ments much smaller for the oral suckers than those of Cercaria douthitti, 

 making it less than half as large as the acetabulum (0.013 mm. to 0.033 

 mm.) Liihe (1909:206) questions his measurements, and since in La 

 Valette St. George's drawing that region is not clear, it may well be that 

 he did not grasp the true proportions of the oral sucker, and that it is as 



